Within the United States, ongoing research is directed toward development of alternative energy sources to reduce our dependence on foreign oil and nonrenewable energy. The use of ethanol as a fuel has become increasingly prevalent in recent years. The current domestic use of ethanol in transportation fuels is about 1.2 billion gallons annually. In the U.S., the major portion of this is derived from the fermentation of cornstarch. Projections made by the Department of Energy indicate that by the year 2020, annual ethanol usage in fuels will have increased dramatically to an estimated 20 billion gallons. This greatly exceeds what can be economically produced from corn starch.
In order to meet the increased demand for ethanol, it will be necessary to ferment sugars from other biomass. Biomass refers to materials such as agricultural wastes, corn hulls, corncobs, cellulosic materials, and the like. Biomass from most of these sources contains xylose at a concentration of up to about 25-30% by weight. A practical, large-scale use must be found for xylose in order for biomass conversion to be economical. Several strains of wild-type or genetically modified yeast are able to produce ethanol through fermentation of xylose, and several bacteria have been genetically engineered for xylose fermentation as well. In general, industrial producers of ethanol strongly favor the use of yeast because yeast are relatively resistant to contamination and are easier to handle in large-scale processing. However, xylose fermentation methods known to the art lack commercial viability.
Xylose is used respiratively by many different yeast species, but it is fermented by only a few species. Fermentation of xylose to ethanol by wild type xylose-fermenting yeast species occurs slowly and results in low yields relative to fermentation rates and ethanol yields that are obtained with conventional yeasts in glucose fermentations. In order to improve the cost effectiveness of the xylose fermentation, it is necessary to increase the rate of fermentation and the ethanol yields obtained.
What is needed in the art is a yeast strain that is capable of fermenting xylose at higher rates to produce greater yields of ethanol relative to that typically obtained by xylose-fermenting yeast strains known to the art.